Antenna tuning apparatus which shorts selected portions of loading coil over complete output cycle



May 9, 1967 w. R. OLSON ANTENNA TUNING- APPARATUS WHICH SHORTS SELECT PORTIONS OF LOADING COIL OVER COMPLETE OUTPUT CYCLE Filed June 28, 1963 2 Sheets-Sheet 1 ,20 RFQPOWER AMPLIFIER UNIT R F POWER GATE24 Fig.2.

GATE 3O May 9, 1967 w. R. OLSON 3,319,168

ANTENNA TUNING APPARATUS WHICH SHORTS SELECTED PORTIONS OF LOADING COIL OVER COMPLETE OUTPUT CYCLE Filed June 28, 1965 E Sheets-Sheet 2 RF POWER MASTER I23 I4 V DRIVER AMPLIFIER OSCILLATOR UNIT H r I Q.

FREQUENCY v ANTENNA CONTROL 7 TUNER UNIT L INlT Fig.4.

RF PowER AMPLIFIER 12 UNIT l8 3, --1 l0 iii i A 2% EIISA $2fi 36A 2,58

'2 Y I37 647 EISC FREQUENCY CONTROL I l UNIT 1 m 9 'i I? se WITNESSES- 1 Fig-5 INVENTOR Wayne R. Olson ATTORN Y United States Patent 3,319,168 ANTENNA TUNING APPARATUS WHICH SHORTS SELECTED PORTIONS 0F LOADING COIL OVER COMPLETE OUTPUT CYCLE Wayne R. Olson, Baltimore, Md., assignor to Westinghouse Electric Corporation, Pittsburgh, Pa., 21 corporation of Pennsylvania Filed June 28, 1963, Ser. No. 291,561 8 Claims. (Cl. 325-172) This invention relates, in general, to means for tuning resonant circuits and more particularly to a system for selectively tuning an antenna system of a very low frequency (VLF) radio transmitter to the instantaneous transmitter frequency.

For such duties as military communications where dependability is of prime importance, very low frequencies of the radio spectrum are used since the ground waves in which the energy is transmitted are but slightly subject to fading and to daily and seasonal variations. The main difficulties encountered in the use of such frequencies have involved the high Q factor (1000 more or less) of antennas of reasonable size. This is for the reason that when the frequency of a driving signal is suddenly changed, the circulating current in a driven circuit consists of a signal at the original frequency which is decaying exponentially from the moment of frequency change, and a signal at the new frequency builds up exponentially at the same moment. In high Q low frequency circuits, the time required for new equilibrium conditions to be established following each frequency change may be so long as to approach the modulation rate, or in frequency shift keyed telegraph systems, the time of a mark or space signal, requiring a decrease in the modulation rate or in the keying speed for avoiding distortion in the transmitted signals. Also, wind, ice, rain loading etc. may affect the tuning of the antenna to the point where the antenna is no longer resonant at the operating frequency of the transmitter with the possibility of grave consequences to the transmitter due to feedback of stored energy in the antenna to the transmitters power amplifier at the wrong time.

Such ditficulties may be overcome by maintaining the antenna resonant frequency substantially the same as the instantaneous transmitter frequency. Such apparatus is broadly disclosed in Letters Patent of the United States, No. 2,712,061 entitled, Means for High Speed Keying at Low Radio Frequency, filed in the name of Cyril E. McClellan and issued June 28, 1955. A non-linear control device such as a saturable resistor may be used as taught by Letters Patent of the United States No. 2,825,030 entitled, Frequency Modulated VLF Transmitter, filed in the names of William Alter and Patrick Conley and issued Feb. 25, 1958. Apparatus for resonantly tuning such an antenna system electronically is illustrated and claimed in Letters Patent of the United States No. 2,679,581, issued May 25, 1954 and No. 2,989,624, issued June 20, 1961, both filed in the name of Mark I. Jacob. All aforementioned patents are assigned to assignee of the present invention.

Whereas Patent No. 2,679,581 utilizes thyratrons to switch in various values of capacitance to tune the antenna, Patent No. 2,989,624 utilizes thyratrons to vary the amount of inductance reflected into the antenna load circuit. They both have the disadvantage of employing electron tubes which take up considerable space and radiate relatively large quantities of heat. Where space requirements are at a minimum and where it is desired to use semiconductor devices exclusively the aforementioned means are not compatible with the present intended usage.

It is an object of the present invention, therefore, to

maintain an antenna circuit at the frequency of a driving signal by changing the inductance of the antenna circuit by selectively shunting a portion of the resonant circuit.

It is another object of the present invention to provide an antenna tuning system for a solid state transmitter wherein controlled rectifiers are operated as switches to resonantly tune the antenna by selectively shunting a portion of the resonant circuit.

It is still a further object of the present invention to provide an improved antenna tuning system whereby controlled rectifiers short-out a portion of the antenna circuit to resonantly tune the radiating antenna to a predetermined operating frequency.

Briefiy, the subject invention comprises a plurality of silicon controlled rectifiers which are used to selectively short-out a predetermined portion of a loading coil connected to the antenna of a solid state transmitter operating in the very low frequency (VLF) region of the electromagnetic spectrum. The silicon controlled rectifiers are operated in pairs such that the current flowing through the loading coil and antenna passes through one controlled rectifier during one-half cycle of the output signal to be radiated and the other controlled rectifier is conductive during the other half cycle. Additionally, the controlled rectifiers are triggered into conduction alternately at a time when the current in the antenna circuit passes through zero allowing a minimum power dissipation within the controlled rectifiers. Other advantages and objects of the present invention will become obvious as the following description proceeds when read in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic electrical diagram of one embodiment of the present invention;

FIG. 2 is an illustration of waveforms helpful in understanding the operation of the embodiment shown in FIG. 1;

FIG. 3 is a schematic diagram of another embodiment of the present invention;

FIG. 4 is a block diagram helpful in understanding the subject invention; and

FIG. 5 is a schematic diagram of still another embodiment of the present invention.

Referring now to FIG. 1, the embodiment illustrated comprises a radiating antenna 22 connected in series cir-. cuit relationship to a loading coil comprised of a pin, ral-ity of coil sections 14 and 15. Magnetically coupled to section 14 of a loading coil 10 is a radio frequency (RF) output coil 12 of a transmitter RF power amplitier 20. One end of the section 15 of the loading coil 10 is returned to a point of reference potential illustrated as ground.

A pair of controlled rectifiers 24 and 30 are connected in parallel across one section 15 of the loading coil 10 to ground. The controlled rectifier 24 comprises an anode electrode 25, a cathode electrode 26 and a gate electrode 27. Similarly, a controlled rectifier 30 comprises an anode electrode 31, a cathode electrode 32 and a gate electrode 33. The parallel connection of controlled rectifiers 24 and 30 is accomplished by connecting the anode electrode to the cathode electrode 32, and connecting the anode electrode 31 to cathode electrode 26. The common connection of anode electrode 31 and cathode electrode 26 is returned to ground, whereas the common connection of cathode electrode 32 and anode electrode 25 is connected to the common terminal between sections 14 and 15 of loading coil 10 by means of a circuit 50.

A transformer 36 is coupled to controlled rectifier 24 for feeding a trigger or gate signal from a control source, not shown to the gate electrode 27 for rendering controlled rectifier 24 conductive. The transformer 36 comprises a primary winding 37 having one end connected to ground while the other end is connected to a terminal 39 to which the trigger signal is applied. The secondary winding 38 of transformer 36 has one end connected to ground while the other end is connected to the gate electrode 27. The primary winding 37 and the secondary winding 38 are poled in such a manner that the application of a positive gate signal to terminal 39 provides a positive trigger signal to the gate electrode 27. The matter of polarities is a matter of choice since it is obvious that the relative polarities would be reversed should it be desired to use a negative gate.

Associated with controlled rectifier 30 is another transformer 40 having a primary winding 41 and a secondary winding 42. As in the case of controlled rectifier 24, the transformer 40 is used to couple a trigger or gate signal from a control source not shown, to the gate electrode 33 of controlled rectifier 30 to render it conductive. The primary winding 41 has one end connected to ground while the other end is connected to a terminal 44 to which the gate signal is applied. The secondary winding has one end connected to the gate electrode 33 while the other end is coupled to the common connection between cathode electrode 32 and anode electrode 25. Transformer 40 has the primary winding 41 and the secondary winding 42 so poled that a positive trigger signal applied to the terminal 44 will apply a positive signal to the gate electrode 33.

In operation the transmitter RF power amplifier 20 delivers RF energy of a predetermined operating or output frequency to the RF output coil 12 which couples the energy to be radiated to the antenna 22 through coil section 14 of loading coil 10.

By selectively shorting out a portion of the loading coil which is in series circuit relationship with the antenna 22, it is possible to tune the antenna system comprising the antenna 22 and the loading coil 10 to the operating frequency of the transmitter. More particularly, controlled rectifiers are operated such that a portion of the loading coil 10 is selectively shunted or shorted out by means of the controlled rectifiers 24 and 30 in a manner which will be explained subsequently. Additionally, if the portion of the loading coil 10 that the controlled rectifiers short out is small compared to the total inductance, the current is essentially a pure sine wave. Since the current waveform in the antenna system is essentially a sine wave, a gate signal is first applied to the gate electrode 27 of controlled rectifier 24 during a first half cycle when the current is flowing in a direction such that it would pass through the anode to cathode junction when controlled rectifier 24 is rendered conductive. For purposes of explanation, this half cycle will be referred to as the positive half cycle. During the negative half cycle, a gate signal is applied to the gate electrode 33 rendering controlled rectifier 30 conductive, shorting the portion 15 during the other half cycle of the output signal. Additionally, the respective gate signals fed to controlled recti-fiers 24 and 30 are such that each is trigger conductive (on) at the time when the inductive voltage is at a maximum and thecurrent in the antenna system is zero. Thus, the energy in the inductor 10 is zero. At the end of the first half cycle, the trinistor 24 shuts off as the current flow therethrough is reduced below its minimum sustaining current, returning the controlled rectifier 24 to its non-conductive state but simultaneously controlled rectifier 30 is rendered conductive to carry the current on the negative half cycle and it remains conductive until the current again becomes zero on its way towards the positive half cycle again. When the inductance of the coil portion 15 of the loading coil 10 is required, the trigger signals applied to terminals 39 and 44 are removed and the controlled rectifiers 24 and 30 remain inoperative over the entire cycle.

Since only a few milliwatts of power are required at the respective gate electrodes 27 and 33 to turn controlled rectifiers 24 and 30 on the control circuitry needed to precisely regulate the tuning of the antenna system can be a matter of choice providing only that it is necessary to use the antenna current as a timing reference so as to ensure that the respective controlled rectifiers are gated on at the peak of the voltage waveform. It is seen that the controlled rectifier is ideally suited to act as the shorting device in the embodiment in FIG. 1. Since the circuit is arranged so that the controlled rectifier 24 is fired when the current in that portion at 15 passes through Zero and is going positive, the controlled rectifier will shunt the current through itself with very little loss. When the current returns to zero, the device shuts off or becomes non-conductive. At that instant, controlled rectifier 34 is fired and carries the current for the remaining half cycle.

Since the controlled rectifier when conducting has a very low voltage drop, typically less than 1 volt, and is able to hold off relatively high voltages, it will have a very high efficiency. As an example, a pair of 50 ampere, 400 volt controlled rectifiers can switch 30,000 voltamperes of reactive power with a loss of only watts, resulting in an efficiency of Reference to FIG. 2 will illustrate the operation of controlled rectifiers 24 and 3th of the embodiment shown in FIG. 1 to short out the portion 15 of the loading coil 10, Curve a of FIG. 2 is a sine wave illustrative of the current wave flowing through the portion 14 of the loading coil 10 and consequently the antenna 22. Curve b illustrates the gate signals applied to terminal 39 and thereupon to the gate electrode 27 of controlled rectifier 24. Curve 0 represents the current flow through the controlled rectifier 24 upon being rendered conductive by means of the gate as shown in curve b. Curve d represents the signal applied to terminal 44 which is coupled to the gate electrode 33 for rendering controlled rectifier 3t) conductive. Associated therewith is curve e which represents the current flow through controlled rectifier 30 which is turned on by means of the gate represented by curve d. Curve 1 represents the current flow through portion 15 of the loading coil 10.

With respect to the curves a through f of FIG. 2, it should be pointed out that the trigger signals, as represented by curves b and d, are applied alternately so that current flows through the respective controlled rectifiers 24 and 30 during each half cycle of the current wave a representing the current flow through the loading coil 14 and the antenna 22. In the absence of any triggers, the resonant frequency of the loading coil-antenna circuit is lower than at the time when the controlled rectifiers 24 and 30 are rendered alternately conductive to short out portion 15 of the loading coil 10. Curve a shows how the resonant frequency of the antenna system is changed due to a shorting effect of the respective controlled rectifiers. Additionally, it should be pointed out that curve 1 shows that during the absence of gates being applied to silicon controlled rectifiers 24 and 30, current will flow through the portion 15 of the loading coil 10. However, upon the application of respective gate signals, the portion 15 is continuously shorted and substantially zero current flows therethrough until the respective gate signals are removed.

If the low losses associated with the controlled rectifiers are not acceptable, they may be reduced to a minimum by means of the embodiment shown in FIG. 3. This embodiment is for all practical purposes identical with the embodiment shown in FIG. 1 with the exception that DC. voltage sources 21 and 23 are inserted in the respective current legs of controlled rectifiers 24 and 30 to offset the small voltage drop occurring thereacross during respective conduction periods. More particularly, the voltage source 21 illustrated as a DC. battery is connected to controlled rectifier 24 such that its positive terminal is connected to lead 50 which is common to the cathode electrode 32 and the common connection between sections 14 and 15 of the loading coil 10.- The voltage source 23 illustrated as a DC. battery is likewise connected to controlled rectifier 30 such that its positive terminal is connected in series with the anode electrode 31 and its negative electrode'is returned to ground.

The operation of the embodiment shown in FIG. 3 is identical to that of the embodiment shown in FIG. 1; however, the power dissipated in the controlled rectifiers 24 and 30 is furnished primarily by the voltage sources 21 and 23, respectively, and not by the resonant circuits comprising the antenna 22 and the loading coil 10.

FIG. 4 is a partial block diagram of a VLF radio transmitter employing the teachings of the subject invention. Briefly, the transmitter comprises a master oscillator 11 which has its operating frequency slaved or controlled by a frequency control unit 13. A signal from the master oscillator 11 is fed to a driver unit 16 which is in turn coupled to a power amplifier unit 20 which, supplies the RF output signal to be transmitted to the RF output coil 12. The RF output coil 12 couples the energy to be radiated to the loading coil and more particularly to the section 14 which is series connected to the radiating antenna 22. A portion 15 of the loading coil 10 has a plurality of taps or terminals to which the antenna tuner unit 18 is connected. Just as the master oscillator 11 is controlled by the frequency control unit 13, so also is the antenna tuner 18 controlled by the frequency control unit 13 to maintain the antenna system resonant at substantially the same frequency as the operating or output frequency of the transmitter. By sending proper signals from the control unit 13 to the antenna unit 18, selective turns of the coil section 15 of the antenna loading coil 10 can be shorted to ground providing the system with a versatility of being able to resonantly tune the antenna system to a plurality of transmitter frequencies. The antenna tuning system associated with FIG. 4 is particularly adapted to frequency shift keying (FSK) type of transmitter operation.

FIG. 5 is an embodiment of the present invention which incorporates the multiple tuning aspect of the present invention as outlined in the block diagram of FIG. 4. Whereas the embodiments of FIGS. 1 and 2 merely provide for one pair of controlled rectifiers, the present embodiment utilizes a plurality of pairs of controlled rectifiers to selectively short out predetermined turns of the portion 15 of the loading coil 10. More particularly, the antenna tuner 18 comprises a plurality of pairs of controlled rectifiers 24A and 30A, 24B and 30B, and 24C and 30C connected respectively across turns 15A, 15B and 15C of the portion 15 of a loading coil 10. Associated with each pair of controlled rectifiers are transformers 40A and 36A, 40B and 36B, and 40C and 36C coupling trigger signals or gates to the controlled rectifiers from the frequency control unit 13. The frequency control unit provides a gate for section A by means of leads 60 and 62 connected to the primary windings of transformers 40A and 36A, respectively, Section B is supplied with gate signals from the frequency control unit 13 by means of leads 64 and 66 connected to the primary windings of transformers 40B and 36B, respectively. And finally, section C is supplied with trigger signals over leads 68 and 70 connected to the primary windings of transformers 46C and 36C, respectively. In operation, the embodiment shown in FIG. 5 operates similar to the embodiments shown in FIGS. 1 and 2 with the exception that one or more, or even all of the pairs, may be activated simultaneously to short out any number of turns of the coil portion 14 of the loading coil 10. When section A comprising controlled rectifiers 30A and 24A is rendered alternately conductive, the turn 15A is shorted out.

Likewise, when the pairs of controlled rectifiers 30B and 24B are rendered alternately conductive, the turns 15B are shorted. It can be seen also that should sections A and B be simultaneously activated, turns 15A and 15B both would be shorted out. It would be obvious then that any combination of these turns could be shorted to provide the required tuning of the antenna to the operating or output frequency of the transmitter. It is a mere design detail whether portion 15 of the loading coil 10 is a single coil with taps or a plurality of separate coils without mutual coupling between them.

What has been shown, therefore, is a system utilizing controlled rectifiers to short out selective turns of an antenna loading coil to properly tune a high Q antenna system to the operating frequency of a VLF transmitter. It is particularly useful in combination with a solid state, high power VLF transmitter having an output power in excess of kilowatts.

Although the present invention has been described with a certain degree of particularity, it should be understood that the present disclosure has been made only by way of example and that numerous changes in the detail of the circuitry in the combination or arrangement of elements may be resorted to without departing from the scope and spirit of the present invention.

I claim as my invention:

1. A system for tuning an antenna for a solid state radio transmitter providing an output signal of changeable frequency comprising in combination: an antenna loading coil operably coupled to said antenna and comprising a plurality of inductors; means for coupling said output signal from said transmitter to said antenna, said means including an inductor of said plurality of inductors; a plurality of semiconductor switching means connected in pairs across selected inductors of said loading coil, with respective semiconductor switching means of said pairs being connected in parallel oppositely poled from one another; and control means coupling said plurality of pairs of semiconductor switching means for selectively rendering said pairs conductive such that a selected number of inductors are shunted over an entire cycle of said output signal for rendering said loading coil and said antenna substantially resonant at said output frequency.

2. The system of claim 1 wherein said plurality of inductors are series connected.

3. The system of claim 1 wherein each of the inductors so shunted has a magnitude of inductance small compared to the total inductance of said antenna loading coil.

4. The system of claim 1 wherein said semiconductor switching means are controlled rectifiers,

5. The system of claim 1 wherein said control means selectively renders said pairs conductive when the current in said antenna loading coil is passing through zero to minimize power dissipation.

6. The system of claim 1 wherein one of each pair rendered conductive by said control means is rendered conductive each half cycle of current through said antenna loading coil and antenna.

7. The system of claim 1 including a direct current voltage source connected to each semiconductor switching means to offset the small voltage drop occurring thereacross when its respective semiconductor switching means is conductive.

8. An antenna system adapted to be tuned to the output frequency of a transmitter of electromagnetic waves comprising in combination: circuit means for coupling electromagnetic energy from said transmitter to said antenna system; a loading coil coupled in said antenna system for providing tuning thereof; and at least one pair of controlled rectifiers connected together in parallel circuit relationship such that the controlled rectifiers comprising said pair are oppositely poled with respect to one another; means coupling said at least one pair across a (I) predetermined portion of lid loading coil, and control References Cited lJy the Examiner 3 52111 33 12325i? 22ii fif ioiiifiifinl ifi 555 UNITED STATES PATENTS trolled rectifiers conductive alternately over a complete 3155922 11/1964 331181 X cycleof an output signal for rendering a selected portion 5 fig giii i t-" 2 5 of said loading c011 inoperative due to the shorting effect 3:193:725 7/1965 Skirpan 307 885 X of said pair of alternately conductive controlled rectifiers, providing a suflicient loading of said antenna to DAVID REDINBAUGH Primary Examiner substantially tune said antenna to the output frequency of Said transmitter 10 B. V. SAFOUREK, Assistant Exammer. 

1. A SYSTEM FOR TUNING AN ANTENNA FOR A SOLID STATE RADIO TRANSMITTER PROVIDING AN OUTPUT SIGNAL OF CHANGEABLE FREQUENCY COMPRISING IN COMBINATION: AN ANTENNA LOADING COIL OPERABLY COUPLED TO SAID ANTENNA AND COMPRISING A PLURALITY OF INDUCTORS; MEANS FOR COUPLING SAID OUTPUT SIGNAL FROM SAID TRANSMITTER TO SAID ANTENNA, SAID MEANS INCLUDING AN INDUCTOR OF SAID PLURALITY OF INDUCTORS; A PLURALITY OF SEMICONDUCTOR SWITCHING MEANS CONNECTED IN PAIRS ACROSS SELECTED INDUCTORS OF SAID LOADING COIL, WITH RESPECTIVE SEMICONDUCTOR SWITCHING MEANS OF SAID PAIRS BEING CONNECTED IN PARALLEL OPPOSITELY POLED FROM ONE ANOTHER; AND CONTROL MEANS COUPLING SAID PLURALITY OF PAIRS OF SEMICONDUCTOR SWITCHING MEANS FOR SELECTIVELY RENDERING SAID PAIRS CONDUCTIVE SUCH THAT A SELECTED NUM- 